Normally, a center frequency in a pass band is stationary in a bandpass filter. In a case where the center frequency is changed in the bandpass filter, there is no alternative but to replace the bandpass filter itself, or to adjust the center frequency again through replacement of an internal element of the bandpass filter.
In such a background, there have been proposed a lot of ideas of a tunable bandpass filter capable of automatically changing the center frequency. However, the tunable bandpass filter requires high cost, a complicated mechanism, and a large scale of peripheral circuitry, and hence the tunable bandpass filter is realizable but has not been often realized actually.
JP-A-2011-9806 (Patent Document 1) discloses the following tunable bandpass filter in FIGS. 1 to 5. Specifically, a metal plate having a ladder shape is provided in a rectangular waveguide, and thus the tunable bandpass filter attains filter characteristics. Further, a dielectric plate is provided in the rectangular waveguide, and a positional relationship of the dielectric plate is changed with respect to the metal plate. Thus, the tunable bandpass filter can change the center frequency. Further, Patent Document 1 further discloses use of a stepping motor as a drive unit for driving the dielectric plate. In addition, Patent Document 1 discloses, in [Claim 6], a matter that the drive unit is controlled by a computer based on predetermined information, to thereby change a relative positional relationship between the dielectric plate and the metal plate.